Calcium carbonate, CaCO.sub.3, occurs naturally in the form of limestone, marble, chalk and coral. Powdered calcium carbonate is produced by either chemical methods or by the mechanical treatment of the natural materials. The term precipitated calcium carbonate applies to the commercial types of the compound produced chemically in a precipitation process. The precipitated products are generally finer in particle size, have a more uniform particle size distribution and a higher degree of chemical purity. A wide variety of calcium carbonate particle sizes and particle shapes can be chemically produced via the precipitation processes. Calcium carbonate is commonly precipitated in the form of calcite, in which the crystals are typically either rhombohedral, cubic or scalenohedral in shape, or in the form of aragonite, which is aciculir. Vaterite is another precipitated form of calcium carbonate known in the art that is metastable. Precipitated calcium carbonate is an extremely versatile filler and pigment that is utilized in a wide variety of manufactured products including paper, paint, plastics, rubber, textiles and printing inks.
Precipitated calcium carbonate (PCC) is used on a large scale in paper filling and coating applications. PCC is utilized to increase the opacity and brightness of paper. In addition to the desirable opacifying and brightening characteristics, PCC provides a high resistance to yellowing and aging of paper. In many high grade coating applications, a fine particle size calcium carbonate is required (median particle size &lt;2 microns). It is typically desirable for the calcium carbonate to be as bright as possible in these high grade coating applications. However, it is difficult to remove the fine dark colored impurities that are introduced by the initial burnt lime source, which is commonly utilized as the raw material in the PCC precipitation process. Such impurities have a negative impact on the brightness and shade properties of the resultant PCC reaction products after processing. More particularly, wet media milling is a common step in the processing of coating grade PCC. It has been found that wet media milling precipitated calcium carbonate generally results in significant loses in pigment brightness due to the grinding of the dark colored impurities present therein. Chemically, a burnt lime is principally CaO, but examples of the impurities commonly found in the burnt lime source include pyrite (iron sulfide), magnesium iron oxides, calcium iron oxides, calcium sulfide and crystalline silicas. As the particle size of the dark particle impurities is reduced through grinding their tinctorial color strength increases dramatically thereby resulting in significant loses in overall product brightness. The loss of PCC pigment brightness from grinding can be on the order of 1.5 to 2.5 points depending on the initial burnt lime source and degree of grinding.
Heretofore, in order to obtain a high quality PCC product with an acceptable brightness, the CaO starting material that is utilized must be of a high quality, i.e. low levels of impurities. If a high quality burnt lime source is not readily accessible, significant logistics costs, about 10%, are added to the cost of the PCC product. Accordingly, the resultant PCC product is relatively expensive.